The World's Greatest Love Letters edited by Michael Kelahan was a little(-ish) book I snagged in the bargain bin for Valentine's Day.
It was a quick read. Only the letters are arranged by subject, which seems really subjective, with no biographical information about the writers or recipients. Which made the reading difficult (and I consider myself decently well-informed regarding British historical figures but Wikipedia only does so much). It also seemed really narrow focus - hetero British exchanges.